It is desirable, in many applications, to adjust the gain levels of a signal. For example, when a signal is transmitted over a line, it may be necessary to increase the gain of the signal to account for degradation in signal clarity and strength. Signal gain may require adjustment (up or down) depending on, for instance, the distance a signal travels from point-to-point. One common device used to adjust signal gain is an automatic gain control (AGC) loop. An AGC loop automatically adjusts the output gain of a signal to a specific level as a function of its input.
In larger systems, it may be necessary to employ several AGC loops connected together to control signal gain. A requirement associated with nesting interconnecting AGC loops is that when one loop is active, the other loop(s) should remain stable to avoid transmitting an unstable signal (garbage data) to the active loop thereby ensuring that the system converges. To verify the stability of an AGC loop, many devices monitor the voltage produced by a charge pump of an AGC loop to determine when the AGC loop is stable. The charge pumps are typically used in the AGC loop to boost or reduce the voltage of a signal to a particular level, then freeze the loop at that level. By monitoring the output voltage (a signal) of the charge pump, it is possible, in principal, to ascertain when an AGC loop is stable.
Determining when the resulting output signal of the AGC loop is stable based on the output voltage of the charge pump, however, can be arduous or impractical. First, it is difficult to detect the stability of the loop by monitoring the output voltage of the charge pump because the final values of the output voltage are often unknown. Second, it is also difficult to detect the stability of the loop by comparing the output voltage of the charge pump because the values of the output voltage are dynamic having differing values even when the loop is stable. Third, to reduce the variations in the output voltage of the charge pump when the loop becomes stable requires a large capacitor (resulting in a large time constant) in the charge pump. As a result, the variations in the output voltage are dampened during both the active and stable periods. Thus, to detect the stability of the loop by monitoring the variations (as the variations are being tempered) in the output voltage with the constraints on the allowable time interval is difficult, if not impossible.
Presently, most detection circuits are design specific. The detection circuits only function with specific AGC loops having predetermined charge pump voltages and time intervals. Other detection circuits are complicated and expensive. Thus, it is difficult and often expensive to design stability detection circuits to operate in a multiple interconnected AGC loop environment, since each AGC loop may have a unique stability characteristic.
Accordingly, what is needed in the art is an improved apparatus that automatically detects when a dynamic signal is stable, without knowing, beforehand, the level and time that is takes a particular dynamic signal to reach stability. Additionally, such an apparatus should employ a simple, low-cost and substantially standardized design (for easy interchangeability in different systems).